1. Field of the Invention
The present invention relates to a voltage controlled oscillator for use in a communication device, and a communication device including the same.
2. Description of the Related Art
A PLL circuit containing a voltage controlled oscillator in a local oscillation circuit is provided, for example, in a communication device such as a terminal, a transceiver, or other suitable device, such as in a mobile communication system such as portable telephones or other suitable mobile communication system.
FIG. 8 shows an example of a conventional voltage controlled oscillator. In FIG. 8, an oscillation circuit includes a transistor Tr2, resistors connected to the transistor Tr2, capacitors, a resonator, a variable capacitance diode VD defined by a voltage variable reactance element. An oscillation signal is output from a buffer stage including a transistor Tr1 via a capacitor C2. The oscillation frequency is changed with an electrostatic capacitance of the variable capacitance diode VD, which is changed via a control voltage applied to a control terminal.
In a high frequency circuit, to suppress effects of undesired radiation from the high frequency circuit and external noises, a high frequency signal is processed in a balance signal mode because this method includes two signal lines defined by transmission lines for balanced signals which equally receive external noises, such that no noises from the balance mode are generated, and also, signals radiated outside from the two signal lines are cancelled.
Such balanced signals are provided by dividing an unbalanced signal into two signals, and shifting the phase of one signal by 180xc2x0 with respect to that of the other signal. Regarding a method of inverting the phase, an emitter follower circuit is provided, and a signal is input to the base of the transistor and is output from the emitter. However, even if an emitter follower circuit which operates in a frequency band of at least 800 MHz used in a mobile communication system is provided, it is difficult to shift the phase exactly by 180xc2x0 in the emitter follower circuit, due to effects of the internal parasitic capacitance and the parasitic inductance of the transistor.
When an output signal from a conventional voltage controlled oscillator as shown in FIG. 8 is processed as an unbalanced signal, it is necessary to convert the unbalanced signal to balanced signals via an unbalance-balance converter including a balun transformer.
Accordingly, the unbalance-balance converter including a balun transformer is disposed in a high frequency circuit section in addition to a voltage controlled oscillator module. Thus, the number of components is increased, and the area occupied by the high frequency circuit section on a printed circuit board or other substrate of a communication device is substantially increased, which increases the overall size of the device.
To overcome the above-described problems with the prior art, preferred embodiments of the present invention provide a voltage controlled oscillator which processes an oscillation output signal as balanced signals without an unbalance-balance converter being separately provided, and a communication device including the same.
According to preferred embodiments of the present invention, a voltage controlled oscillator includes an oscillation circuit having an active element and a voltage variable reactance element provided on a ceramic multilayer substrate, and an unbalance-balance conversion circuit mounted onto the upper surface of the ceramic multilayer substrate. Thereby, the voltage controlled oscillator including the ceramic multilayer substrate and balance-outputs oscillation signals is provided as a single component.
Moreover, according to preferred embodiments of the present invention, a voltage controlled oscillator includes an oscillation circuit having an active element and a voltage variable reactance element provided on a ceramic multilayer substrate, and an unbalance-balance conversion circuit defined by transmission lines sandwiching a desired ceramic layer of the ceramic multilayer substrate. Thereby, the desired layer of the ceramic multilayer substrate operates as the unbalance-balance converter. In the voltage controlled oscillator having the above-described configuration, no unbalance-balance conversion element is required to be mounted on the component-mounting surface of the ceramic multilayer substrate. Thus, a separate mounting area is not needed for the unbalance-balance conversion element. Therefore, the size of the component is greatly reduced.
In the voltage controlled oscillator of other preferred embodiments of the present invention, a plurality of sets of the oscillation circuit and the unbalance-balance conversion circuit, corresponding to each frequency, are provided on the ceramic multilayer substrate.
A conventional voltage controlled oscillator is provided with a single oscillation output terminal. On the contrary, with the configuration according to preferred embodiments of the present invention, oscillation signals corresponding to frequency bands and to be balance-output are used.
The communication device of other preferred embodiments of the present invention is provided with the voltage controlled oscillator having one of the above-described configurations, for example, by providing a PLL circuit which is a local oscillation circuit.
The features, characteristics, elements and advantages of the present invention will be clear from the following detailed description of preferred embodiments of the invention in conjunction with the accompanying drawings.